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Science: Winking stars could reveal Galaxy's black hole

29 February 1992

By
KEN CROSWELL in
BERKELEY

If a massive black hole lies at the heart of our Galaxy, its gravity
should distort, or ‘lense’, the light from stars that lie behind it, say
two astronomers in the US. This effect would be detectable by a future generation
of infrared and radio telescopes.

The centre of the Galaxy is about 27 000 light years from Earth. Concentrated
in a volume only a few light years across is an amount of material with
several million times the mass of the Sun. No one knows what makes up this
mass.

The centre of the Galaxy is marked by a peculiar source of radio waves
known as Sagittarius A*, which may be a massive black hole containing millions
of solar masses. Such a black hole would account for most of the mass at
the galactic centre. The other possibility is that Sagittarius A* is less
massive and that a dense cluster of normal stars makes most of the mass
at the Milky Way’s centre.

Now Mark Wardle and Farhad Yusef-Zadeh of Northwestern University in
Illinois say that there may be a way to tell which is the truth. The method
they propose makes use of a phenomenon called gravitational lensing, in
which the gravity of a massive object – such as a black hole – splits the
light of a star behind it into two or more separate images.

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Sagittarius A* lies in a star cluster. If Sagittarius A* really is a
massive black hole, it should gravitationally lense several of the stars
which lie behind it, say Wardle and Yusef-Zadeh. Each lensed star will be
split into two images, which will appear on opposite sides of Sagittarius
A* as viewed from Earth. Although each image will look like a normal star
that is not being lensed by the black hole, the researchers think there
could be a way of telling whether gravitational lensing is really taking
place.

All stars in the cluster are moving, because they orbit around the centre
of the Galaxy. The relative positions of a lensed star and the black hole
will therefore change. Wardle and Yusef-Zadeh estimate that this change
in position will in turn cause the brightness of the two images of the lensed
star to rise and fall together over a period of a few months or a few years.
Observing this phenomenon would indicate that lensing is taking place.

The best wavelength at which to observe such lensing would be the infrared,
say Wardle and Yusef-Zadeh. Infrared light from the stars in the cluster
that harbours Sagittarius A* can penetrate the thick clouds of gas and dust
surrounding the Galactic centre. Visible light is absorbed by these dust
clouds.

To detect such effects, Wardle and Yusef-Zadeh say that infrared telescopes
must be able to separate stars that are only 10 milliarcseconds apart –
the equivalent to being able to pick out a human being from a distance of
35 000 kilometres. Present day infrared telescopes do not have the ability
to resolve enough detail, but it should be possible some day.

Wardle and Yusef-Zadeh also suggest another test for a massive black
hole at the Galaxy’s centre. If it exists, the black hole should gravitationally
lense radiation from a radio source named Sagittarius A East, which lies
hundreds of light years behind Sagittarius A* and slightly to one side of
it. If Sagittarius A* is a massive black hole, a smaller image of Sagittarius
A East should appear on the other side of Sagittarius A*.

According to Wardle and Yusef-Zadeh, such an image might be detectable
with a radio telescope now being built called the Very Long Baseline Array,
which will be made up of a series of radio antennas at various locations
between the Caribbean to Hawaii. The Very Long Baseline Array will be completed
within a year or so.

Wardle and Yusef-Zadeh will publish their predictions in a future issue
of Astrophysical Journal Letters.